Chapter 17 - 17-2Chemical thermodynamics: the study of the...

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Unformatted text preview: 17-2Chemical thermodynamics: the study of the energetics of chemical reactions17-3The following concepts and terms have been discussed in Chapter 5 and should be reviewedSystemSurroundingsState functionEnthalpyStandard stateStandard enthalpy of formationHesslawHeat capacityHeat17-4Thesystemis a sample of matter, on which we focus our attention, generally the chemical reaction.The surroundingsare all other matter in the universe.Enthalpy is a measure of the total energy possessed by a system at constant temperature and pressureThe change in enthalpy (H)is the heat absorbed (released) by the system at constant pressure and temperatureHessLawallows the calculation of the H of a reaction from measured enthalpy changes for other reactions.Heat (q) is the energy that causes a change in the thermal energy of a sample. Addition or removal of heat from a sample causes a change in temperatureAstate functionis any property of a system that is Fxed by the present conditions and is independent of the systems history.We deFne the standard stateof a substance at a speciFed temperature as its pure form at 1 atm pressure.The heat capacityof an object is the quantity of heat required to raise the temperature of that object by 1 K. The units are J/K.17-5Under normal laboratory conditions, a reaction (the system) usually exchanges energy with the surroundings in two forms, as workand as heat.Work is directed motionHeat is random motion17-6Workis the application of a force through a distancew= force x distanceWhen a gas expands against an opposing pressure work is performedw= -PVUnit of Work: Joule(Kgm2)/s2R=0.0821LatmmolK=8.314JmolK0.0821Latm=8.314J1Latm=101.3J17-7Express the work (in Joules) when 20.0 L of an ideal gas at a pressure of 12.0 atm, expands at constant temperature against a constant pressure of 1.50 atm.17-8Use Boyles law to fnd the fnal volumeV2=P1V1/P2= 20.0 L x 12.0 atm/1.5 atm= 160 LCalculate the work and convert Latm to Jw= -PV= -1.50 atm x (160 - 20.0)L= -210 Latm x 101.3 J/Latm = -2.13 x 104J17-9First law of thermodynamicsis the law of conservation of energy; energy can be neither created nor destroyedInternal energy, E,represents the total energy of the system and is a state function(it depends only on the initial and Fnal state of the system)17-10When a change occurs in a closed system, the change in internal energy is given byE= q+ wThe sign convention for qand ware:qand ware positiveif they transfer energy to the system from the surroundingsqand ware negativeif they transfer energy from the system to the surroundings 17-11When a reaction takes place at constant volume and temperature, no PVwork is performed so E= qThe heat measuredwith a constant volume calorimeter equals the change in internal energy (E) of the system17-12...
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Chapter 17 - 17-2Chemical thermodynamics: the study of the...

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